f2fs: make exit_f2fs_fs more clear

init_f2fs_fs does:
    1) f2fs_build_trace_ios
    2) init_inodecache
    3) create_node_manager_caches
    4) create_segment_manager_caches
    5) create_checkpoint_caches
    6) create_extent_cache
    7) kset_create_and_add
    8) kobject_init_and_add
    9) register_shrinker
    10) register_filesystem
    11) f2fs_create_root_stats
    12) proc_mkdir

exit_f2fs_fs should do cleanup in the reverse order
to make the code more clear.

Signed-off-by: Tiezhu Yang <kernelpatch@126.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>